My experience is, yes, they work - for small punctures. Goat-heads, cacti, or anything that puts a small hole in your tube can be fixed with self-sealing tubes. Big cuts, tears, etc., well, it don't work for that. I live in an area prone to small punctures, but I don't use "Slime" tubes. Instead, I run good DH tires (Maxxis, Kenda) that have built-in puncture resistant inserts, and thick DH tubes. I can kack a curb or rock or run over a cactus with no problems. In fact, up until about a month ago, I hadn't had a flat for over two years, and the only reason I did is because I ran over a huge-ass piece of glass from a bottle that some crack-head broke on the street. Slime tubes aren't worth the money, mess, or extra rolling weight (IMO), but if you're going to get some, buy the bottle of slime and fill your own tubes instead of paying extra cash for the ones that at "pre-slimed". It takes a bit extra work to fill presta tubes, but if you're riding DH, you probably have schraeder tubes, so it makes no difference.

One of my greatest riding memories was at Big Sky, MT. Some cheesedick decided to try a little DH on his Mongoose MGX. 30 yards down the first run he hits a big piece of andesite shale, destroys his front wheel and gets covered with spewing green slime. Sweeeeet. Bye bye Mr. MGX.

I ran tubeless 3 years ago and loved how they rolled, but hated the fact that whenever I jumped, they made raunchy farting sounds, leaving my tires low on air. The other disadvantage to tubeless is that if you get a large hole, you're fukced, which was my experience when I tore out a sidewall on a sharp root. Luckily I had a spare tube, a dollar bill (to cover the hole), and a tire pump. Of course, instead of just going out and buying a $5 tube, I had to get a whole new tire...lame, lame, lame...